1. Field of the Invention
This invention relates to a method for the production of phthalic anhydride by the vapor-phase oxidation of a mixture of ortho-xylene with naphthalene. More particularly, it relates to a method for the production of phthalic anhydride by catalytic vapor-phase oxidation effected by introducing a mixture of ortho-xylene with naphthalene and a molecular oxygen-containing gas into a shell-and-tube type fixed-bed reactor packed with a specific catalyst.
2. Description of the Prior Art
As a means of producing phthalic anhydride from the mixture of ortho-xylene with naphthalene as a raw material, a method which, for example, uses a catalyst based on a vanadium-titanium dioxide composite (JP-A-58-74675) has been known to the art. The method disclosed in this patent publication comprises using a catalyst for oxidation of ortho-xylene until the catalyst has been deactivated through aging and then introducing naphthalene into the site of oxidation in proportion to the degree of deactivation. This introduction of naphthalene is effected particularly after the reaction of ortho-xylene only has continued for 50 months. This method remains inexecutable for a considerable time until the feed of the mixed raw material is able to start. Further, it does not allow selection of a mixing ratio of raw materials which is economically advantageous in the light of the existing situation of the supply of raw materials. This method cannot generally be embodied because it does not define the catalyst in terms of such factors as chemical composition.
An actual case of successful production of phthalic anhydride from a mixture of ortho-xylene and naphthalene of a freely selected ratio is reported in "Aromatics," Vol. 38, Nos. 9-10, pages 12-18 (1985). Again in this case, however, the production is performed after the reaction exclusively of naphthalene has been continued for one year. The report has no mention of the possibility of this method being performed from the outset of the reaction. It offers no detailed description of the catalyst to be used in the method and, therefore, does not allow identification of the catalyst.
U.S. Pat. No. 4,879,387 is known to have disclosed a catalyst composition and a method for the use thereof. It discloses a catalyst for the oxidation of naphthalene and also discloses working examples using a 50/50 mixture of ortho-xylene/naphthalene (Examples 3 and 27). It further discloses working examples for effecting a reaction solely of naphthalene with the same catalyst as already used for a reaction solely of ortho-xylene (Examples 26 and 10). Though this patent publication does contain a mention, to the effect that, naphthalene and ortho-xylene may be used as a mixed raw material, it does not show any measures which cope with changes in the hot spot (the spot of the highest heat in the catalyst bed) which occur in the use of the mixed raw material resulting from the difference in the mixing ratio.
The inventor of U.S. Pat. No. 4,879,387 mentioned above, relating to the production of phthalic anhydride by the use of a mixed raw material, offers a detailed description of a method for mixing the raw material in JP-A-1-190677. This patent publication, however, shows no restriction regarding the catalyst. A review of the working examples cited in this patent publication leads to an inference that the method disclosed is usable with all the mixing ratios ranging from 100% of ortho-xylene to 100% of naphthalene. The yields of produced phthalic anhydride indicated in these working examples, however, are considerably lower than those indicated in the working examples cited in U.S. Pat. No. 4,879,387.
As shown above, the catalyst which is usable for all mixing ratios must be capable of coping with changes in the mixing ratio by sacrificing yield or product as compared with a catalyst which is optimized for the mixed raw material having a fixed mixing ratio.
We have so far studied and developed catalysts for the production of phthalic anhydride by the oxidation of ortho-xylene and/or naphthalene (JP-A-56-73543, JP-A-56-78635, and JP-A-57-105241) to realize high productivity (high load and high selectivity) and long catalyst service life in a limited range of mixing ratios. When the conventional catalyst is used, the mixing ratio of the components of the mixed raw material mentioned above is limited to a narrow range. When the mixing ratio deviates from this range, the production entails numerous problems relating to the yield of phthalic anhydride produced, the quality of the product, and the service life of the catalyst. These problems are prominent particularly when the reaction of oxidation is carried out with a catalyst which is favorable for the oxidation of naphthalene and the proportion of ortho-xylene in the mixing ratio of ortho-xylene to naphthalene is not less than 50% or when the reaction is carried out with a catalyst which is favorable for the oxidation of ortho-xylene and the proportion of ortho-xylene in the mixing ratio of ortho-xylene to naphthalene is not more than 50%.
Specifically when the reaction is carried out with a catalyst which is favorable for the oxidation of naphthalene and the proportion of ortho-xylene in the mixing ratio of ortho-xylene with naphthalene is not less than 50%, the amount of phthalide, a substance conductive to adverse effects in the quality of the product, and which is generated in the reaction, increases notably.
Conversely, when the reaction is carried out with a catalyst which is favorable for the oxidation of ortho-xylene and the proportion of ortho-xylene in the mixing ratio of ortho-xylene to naphthalene is not more than 50%, the reaction temperature must be lowered to ensure a high yield of the reaction. Again in this case, the amount of naphthoquinone, a substance detrimental to the quality of the product and known to be generated, is increased. When the reaction temperature is heightened for the purpose of repressing the occurrence of this mischievous substance, the yield of phthalic anhydride is impaired and an abnormal hot spot occurs in the frontal part of the catalyst bed and produces an adverse effect on the service life of the catalyst. These problems are liable to gain further prominence when the catalyst is exposed to a high load.
An object of this invention, therefore, is to provide a novel method for the production of phthalic anhydride by vapor-phase oxidation of a mixture of ortho-xylene with naphthalene.
Another object of this invention is to provide a method which produces phthalic anhydride of high quality from a mixture comprising of ortho-xylene and naphthalene in a widely variable ratio over a long period even when the load exerted on the raw material is high and also provide a catalyst composition useful for the execution of the method.